This invention relates in general to drive circuits for laser apparatus and, more particularly, to pulse width modulation drive circuits for pulsed diode lasers.
Before discussing a conventional laser diode drive circuit, a conventional drive circuit 10 for a flashlamp-type laser transmitter 15 is shown in FIG. 1 for comparison purposes. The laser transmitter includes a flashlamp which excites a laser rod (both not shown). The transmitter also includes an energy monitor 20 which generates an ENERGY DATA output signal representative of the amplitude of the laser transmitter's optical output. The ENERGY DATA signal is fed through a transmitter electronics module 25 as an ENERGY STATUS signal. The ENERGY STATUS signal is supplied to a laser electronics module 30 which generates a high voltage adjust signal (HV Adjust). The HV ADJUST signal is fed to a programmable high voltage power supply, the output PROG. HV signal (pump voltage level) of which varies in accordance with the supplied HV ADJUST signal. The PROG. HV output signal (pump voltage level) is supplied to a pulse forming network (PFN) 40 including a capacitor in transmitter electronics module 25. The output of pulse forming network 40 supplies an ionization/PFN signal to laser transmitter 15 to drive the flashlamp exciter therein.
The drive circuit of FIG. 1 employs a control loop to adjust the output of the laser transmitter in the manner now described. The optical energy output of the laser transmitter is dependent on the pump voltage level (PROG. HV) supplied to the pulse forming network capacitor in the transmitter electronics module. The pump voltage level (PROG. HV) is programmed by, or controlled, by the present value of the HV Adjust signal generated by the laser electronics module. More specifically, using ENERGY DATA information sent back from the energy monitor in the laser transmitter (via the ENERGY DATA serial word data), an algorithm in the laser electronics module determines what adjustment of the pump voltage level (PROG. HV), if any, is needed to maintain a constant desired level of output energy. Thus, in this particular example wherein the laser transmitter includes a flashlamp cavity box, the optical output of the laser transmitter is maintained at a relatively constant amplitude by appropriately varying the pump voltage or drive voltage which is supplied to the laser flashlamp.
In contrast to the above described control loop of FIG. 1 wherein voltage is used to control the output of the flash lamp type laser transmitter, FIG. 2 shows a control loop and drive circuit which employs current to control the output of a laser diode type laser transmitter. FIG. 2 shows a laser diode driver circuit including an operational amplifier 100, the output of which is coupled to drive a field effect transistor 105 which is coupled in series with a laser diode 110, an energy storing capacitor 115 and a resistor 120. As noted above, one conventional approach to varying the output energy of the laser diode is to vary the amplitude of the current supplied to the laser diode. Another conventional technique for varying the output energy of the laser diode is to vary the pulse width of the input drive signal supplied to the input of the drive circuit at operational amplifier 100. Within certain bounds, the longer the pulse width supplied to the laser diode, the greater is the output which is generated by the laser up to the point of saturation. This is called pulse width modulation.
Unfortunately, conventional laser diode drive circuits such as shown in FIG. 2 do not enable the laser to stabilize quickly enough upon startup for some applications such as laser designator rangefinders, for example. Moreover, such drive circuits do not compensate for aging effects of the laser diode or other irregularities which enter the laser system. Also, temperature effects cause laser diode output wavelength shifts which, in general, decouple from the absorption region of the lasing medium (Nd:YAG rod) resulting in lower laser system output. Optics degradation, cooling medium "fogging" (if cooling the volume between laser diodes and lasing medium), and thermal effects on the lasing medium can result in undesired laser system output energy shifting when using laser diode drive circuits such as that in FIG. 2.